Microsoft Security Response Center (MSRC)
Programmatic interfaces to engage with the Microsoft Security Response Center (MSRC)

Mozilla observatory http scanner.
Mozilla http scanner API is a free Security API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Mozilla HTTP Observatory.
Details from github.com.

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Programmatic interfaces to engage with the Microsoft Security Response Center (MSRC)
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The Mozilla http scanner API is provided by mozilla (github.com). Mozilla HTTP Observatory.
Yes. The Mozilla http scanner API is listed in the Security category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Mozilla http scanner API requires no authentication, so you can start making requests without signing up.
CORS support for the Mozilla http scanner API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Mozilla http scanner API is served over HTTPS, so it's safe to use in production.